An international working group coordinated by Forschungszentrum Jülich has completed a new mirror system for ITER, which protects optical components from being contaminated by particles in the vacuum vessel. It is now being tested at the DIII-D tokamak in the US.

Optical measurements are indispensable for nuclear fusion experiments. The light produced in a plasma speaks volumes about its density and temperature, its composition and the concentration of various isotopes and elements. But ITER and future fusion experiments will burn tritium-deuterium plasmas, whose intense neutron radiation will degrade light-guiding fibres and damage sensitive detectors. Therefore it will only be possible to observe the light indirectly via mirror systems. The mirrors, however, could become contaminated by beryllium and tungsten particles originating from eroded wall materials,which would degrade their reflectivity .

The new concept for ITER involves a system of ducts with mirrors at their ends. This so called mirror station is equipped with shutters which only uncover the mirrors during the main phase of the plasma pulse. These shutters are closed during the ignition of the plasma when the contamination risk is highest. Since the very strong magnetic fields in the vacuum vessel interfere with electrical circuits, the shutters rely on passive control. Once the plasma ignites, it generates a magnetic field which acts on magnetic ferrite cores in the shutters and triggers them to open. Several ducts of the mirror station are trialling special fins to protect the mirrors from incoming particles during the measurements. This would further extend the mirror lifetime.

Dr Andrey Litnovsky, FZ Jülich
a.litnovsky AT fz-juelich DOT de